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Cash, cash equivalents and short-term investments (Details) (USD $)
 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 18 Months Ended
Jun. 30, 2013
item
Jun. 30, 2012
Jun. 30, 2013
item
Jun. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Jun. 30, 2013
Corporate debt securities due in one year or less
Dec. 31, 2012
Corporate debt securities due in one year or less
Jun. 30, 2013
Corporate debt securities due in two years or less
Dec. 31, 2012
Corporate debt securities due in two years or less
Dec. 31, 2012
Marketable securities
Jun. 30, 2013
Short-term investments
Jun. 30, 2012
Short-term investments
Jun. 30, 2013
Short-term investments
Jun. 30, 2013
Short-term investments
Jun. 30, 2013
Financial institution debt securities and other corporate debt securities located in Canada, the United Kingdom, the Netherlands, Australia, and Norway
Cash, cash equivalents and marketable securities                                
Cash and money market funds, Amortized Cost $ 28,193,000   $ 28,193,000   $ 14,776,000                      
Cash and money market funds, Fair Value 28,193,000   28,193,000   14,776,000                      
Amortized Cost 54,371,000   54,371,000   31,397,000   20,251,000 5,754,000 5,927,000 10,867,000 16,621,000 26,178,000   26,178,000 26,178,000 13,300,000
Gross Unrealized Gains 1,000   1,000   5,000   1,000 2,000   3,000 5,000 1,000   1,000 1,000  
Gross Unrealized Losses (27,000)   (27,000)   (11,000)   (16,000) (1,000) (11,000) (10,000) (11,000) (27,000)   (27,000) (27,000)  
Fair Value 54,345,000   54,345,000   31,391,000   20,236,000 5,755,000 5,916,000 10,860,000 16,615,000 26,152,000   26,152,000 26,152,000  
Amortized cost of cash and cash equivalents and marketable securities 54,588,000   54,588,000   31,397,000                      
Fair value of cash and cash equivalents and marketable securities 54,561,000   54,561,000   31,391,000                      
Amortized cost of cash and cash equivalents 28,193,000 34,067,000 28,193,000 34,067,000 14,776,000 4,541,000                    
Fair value of cash and cash equivalents 28,193,000   28,193,000   14,776,000                      
Number of debt securities that had been in an unrealized loss position for less than 12 months 21   21                          
Number of debt securities that had been in an unrealized loss position for 12 months or longer     0                          
Fair value of debt securities that had been in an unrealized loss position for less than 12 months 20,200,000   20,200,000                          
Realized gains                       0 0 0 0  
Realized losses                       0 0 0 0  
Other-than-temporary impairments recognized $ 0 $ 0 $ 0 $ 0